text:
What determines the positioning of PAPI lights from the runway threshold?
Why are PAPI lights installed 300m from the threshold?
What are the conditions that define the position of the PAPI lights from threshold?

US DoT Order JO 6850.2B contains installation criteria for visual guidance lighting systems. The following is taken from that order.
For siting PAPI along the runway, in case an (ILS) electronic glide slope is present, the lights are located such that the PAPI visual approach path coincides with the one produced electronically as much as possible.
In case of runways without an electronic glide slope, the following criteria are considered for locating the PAPI:

Threshold Crossing Height

The TCH is the height of the lowest on-course signal at a point directly above the intersection of the runway centerline and the threshold. The minimum allowable TCH varies according to the height group of aircraft that use the runway, ... The PAPI approach path must provide the proper TCH for the most
  demanding height group that uses the runway.

Glide Path Angle

The visual glide path angle is the center of the on-course zone, and shall normally be 3 degrees when measured from the horizontal.

Obstacle Clearance Surface

The PAPI obstacle clearance surface is established to provide the pilot with a minimum clearance over obstacles during approach. The PAPI must be positioned and aimed so that no obstacles penetrate this surface.

Image from US DoT JO 6850.2B Visual Guidance Lighting Systems
Basically, in case ILS is there, align the PAPI with that; else locate the PAPI such that it satisfies TCH and OCS.

Excellent answer; I knew there would be some sort of technical order out there for this. However, I would question that given the presence of an ILS GP, the PAPI will be coincident; many are not. Take the KMKT ILS 33 for example. See the "VGSI and ILS glidepath not coincident" note in the profile view. See also KGEG ILS 21, KLVN ILS 30, etc. These are just ones I have personally experienced, but there are many others.

Practically (rather than just by the TO), there are three indications that should coincide 1,000ft from the threshold. This answer of course mentions the two in the TO: PAPI and the GS. The third is the runway markings, particularly the aiming point as per http://www.code7700.com/images/runway_aim_touchdown_markings.png

@rbp Technical specs for aiming point markers can be found in AC 150/5340-1L

Additonal reference containing technical specs for PAPI: AC 150/5340-30H

good references. but not my point.

(Image Source: WikiPedia)
In this picture, you see that the PAPI is placed right next or near to the touchdown zone.

(Image Source: Aviation.SE)
Since you want guidance for this area, placing them near the threshold would actually make you aim too close to the threshold and increase the possibility of touching down too short of the runway.

Overrun that runway and you go right into the drink! That's quite demanding!   ;)


text:
How are the frequency pairings for localizer and glideslope decided?
How are the frequency pairings for localizer and glideslope decided?

As always the country is responsible for the final decision, so FAA/FCC will decide in the US (but see @Gerry 's comment from an insider below), and I believe so does the MIC in Japan. However this local decision happens after an agreement already found in international bodies. For aviation, the international body is ICAO and ICAO Chicago Convention is the main framework used to ensure international aviation operations are possible, safe and fair:
The localizer / glide path pairing table is part of Annex 10 of the Convention, Volume I, paragraph 3.1.6.

3.1.6.1 The pairing of the runway localizer and glide path transmitter
  frequencies of an instrument landing system shall be taken from the
  following list in accordance with the provisions of Volume V, Chapter
  4, 4.2.

ICAO decisions in radiocommunication matters are subject to the frequency allotment and protection decisions taken under the auspices of the ITU during the World Radiocommunication Conference (WRC), held every four years. It's ITU which is responsible for the global (civil) use of the frequency spectrum.

The pairing table was built back in 1956! (Fifth Session of the COM
Division, as visible on p. XI)
Pairing was carefully designed so that ILS could work without interferences (harmonic and heterodyne) in certain conditions without being below the radio horizon of each other.
A preferred pairing table is provided for such use:

3.1.6.1.1 In those regions where the requirements for runway
  localizer and glide path transmitter frequencies of an instrument
  landing system do not justify more than 20 pairs, they shall be
  selected sequentially, as required, from the following list:

Attachment C of Annex 10, (2.6 Deployment of ILS frequencies) provides some details about this selection:

2.6.5 In general, when international use of ILS systems is confined to the pairings listed in Chapter 3, 3.1.6.1.1, the criteria are such
  that, provided they are met for the localizer element, the glide path
  element is automatically covered. At certain congested locations,
  where it is necessary to make assignments in both the first ten and
  the second ten sequence pairings, it may be necessary to select
  certain pairings out of sequence in order to meet the minimum
  geographical separation in 2.6.6.
Example: Referring to Chapter 3, 3.1.6.1.1, it will be noted that ILS
  Sequence Number 2 pairs the localizer frequency of 109.9 MHz with
  glide path frequency 333.8 MHz. Sequence Numbers 12 and 19, however,
  although providing wide frequency separation from Sequence Number 2 in
  the case of the localizers, assign frequencies of 334.1 MHz and 333.5
  MHz, respectively, for the glide paths, both being first adjacent
  channels (300 kHz spacing) to the Sequence Number 2 glide path
  channel. If selection of ILS channels is confined to either the first
  ten or the second ten pairings, then the minimum glide path frequency
  separation will be 600 kHz.

Your statement "FCC will decide in the US" is not entirely accurate. The NTIA is the US representative to the ITU and the NTIA Office of Spectrum Management has the primary responsibility for managing that spectrum. They allocate spectrum to other agencies. The "Aeronautical Radionavigation Bands" have been allocated to the FAA who has their own office of spectrum management.  The FCC has limited influence over the spectrum allocated to the FAA mostly related to interference. Having dealt with these offices in the past, I can say "It's complicated."

@Gerry: Thanks for that comment, I was not aware of that, though it's quite natural that several agencies will actually deal with specific sectors.

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text:
What is the effect of wind on altimeter reading?
I am wondering is there any effect from wind on the altimeter reading?
I think when the wind is blowing, the static pressure around the aircraft in flight will somehow change and this can impact the altimeter reading. Is it correct or wrong?

Steady wind has no aerodynamic effect on an airborne aircraft in any way. "Wind" is the motion of the airmass relative to the ground, but the aircraft isn't connected to the ground. The aircraft is physically connected only to the airmass.

A steady wind will not effect the altimeter reading on an airborne aircraft in any way, since the aircraft will be moving in relation to the airmass. This fully removes any effect a steady wind will have on the airframe and pressure sensitive systems attached to it.
Gusts may have temporary effect on altimeter reading depending on the location of the static port, a sudden gust may alter the pressure distribution around the airframe for a very short period of time inducing a higher or lower than actual pressure reading.

Not only can the actual pressure distribution change, but the wind gust can blow into the static port and thus show the dynamic pressure caused by the gust wind velocity instead of the actual (=static) pressure. That is actually a real problem when measuring turbulent pressure fluctuations in turbulent environment, the sensors will pick up a lot of noise from the dynamic pressure.

Yes, if a gust of wind was blowing directly into a static port, it could cause a temporary increase in measured static pressure and a drop in the altitude readout.
Traditionally this was corrected by putting static ports on both sides of the aircraft and connecting both of them together, in a T layout. This reads the average pressure on both sides.
Modern large aircraft instead use digital sensors on both sides and the air data computers combine the data. This eliminates potential leakage in the piping, which has caused gross altimeter errors in the past.
Large aircraft combine data from multiple sensors, particularly accelerometers from the inertial nav system, and AOA and sideslip probes. These allow corrections to be made for gusts and local airflow, important to be able to meet altimetry accuracy requirements for RVSM and Baro-VNAV. Another trick is that at least one sensor will be placed asymmetrically, so that vertical gusts will have a unique signature.
This is why air data functions are in the same box as the inertial reference units, forming the integrated air data inertial reference system (ADIRS).

static pressure around the aircraft will somehow change, and this can impact the altimeter reading

At the worst possible moment, when trying to hold a ground track
in a landing pattern.
When holding a ground track, effects of wind direction can produce a relative wind that is not aligned with the direction of flight.
This could produce a wind sideforce on the aircraft, potentially affecting the static tube pressure by increasing (windward) or decreasing (leeward) "static" pressure.
Designers solve this by placing static ports in areas less affected by sideslip and/or gusts, and some, like the Cessna 172, have more than 1 static port, giving an average static pressure.
Any blockage, such as by icing, could also play havoc with the altimeter.
The Cessna 172 has 2 static ports, on either side of the fuselage, near the engine cowling.

Re "When holding a ground track, effects of wind direction can produce a relative wind that is not aligned with the direction of flight."-- not if a) the wind is steady in speed and direction (and also is steady across the altitude spectrum, if the plane's altitude is not constant), and b) the pilot initially gets the plane onto the proper ground track by making a coordinated turn as needed until the ground track is correct, and then levelling the wings and relaxing all control input and trimming for normal coordinated flight.  Sure, gusts can create transient side-wind components.

But, just to be clear, I didn't DV (yet!  ; )  )

A side slip approach holding the nose in line with the runway is a case to be considered.  Gusts and changes in wind are also more prevalent closer to the ground.  Any how about slipping with an engine out?  Yes, we would all like to be coordinated in a smooth steady wind, but it isn't always that way.

That close to the ground, the pilot shouldn't be too fixed on a pressure altimeter (allowable error, 75'). A radio altimeter, maybe - for Cat II or Cat III ops. The question is only about effects of wind, not slipping, engine-out, etc.

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text:
Is collaring a circuit breaker considered deactivation by the FAA?
Let's say you discover that your landing light is inoperable during the preflight. Your aircraft doesn't have a MEL, so you follow 14 CFR §91.213(d). Assuming you do everything else required (placard, etc.), would placing a collar around a circuit breaker be considered deactivation?
If not, is there anything that a private pilot can do that would be considered deactivation?

The humorous absurdity of pulling & collaring a breaker (interrupting the circuit) for a failed landing light (usually because the incandescent filament has burned out, interrupting the circuit) is not lost on me. Though on a serious note, if we're talking about a fault in a HID landing light system or LED landing light the potential failure modes certainly warrant "preventing the operation" of the system until you know what caused the fault...

It does seem a bit absurd. I asked this question because it's one that seems to come up in checkrides. During my checkride I was asked what I would do in this situation. I answered that I would collar the circuit breaker, and the DE told me that would not be enough and I would have to have a certified mechanic repair the light or disconnect the power to the circuit breaker to be legal. I didn't argue, but I'm pretty sure he's wrong and the collaring is deactivation.

I would think collaring qualifies as deactivation too - certainly as lnafziger pointed out it's what manufacturers recommend, and I'm pretty sure it's what airlines do for non-critical equipment that needs to be "tagged out" because it's broken. For that matter taping a bent index card cover over the switch so you can't turn it on it would also seem to qualify, albeit a somewhat inelegant solution.

Based on this Advisory Circular, the answer is yes.

g. Deactivation means to make a piece of equipment or an instrument unusable to the pilot/crew by preventing its operation

Collaring the circuit breaker prevents it from being pushed in, and therefore prevents its operation.

This is consistent with MEL M&O procedures that in some cases specify doing exactly this.

Per AC 91-67A, there is nothing a private pilot can do:

Deactivation. When an item is “deactivated” or “secured,” or both, the specified item
must be put into an acceptable condition for safe flight. Deactivation may involve more
than simply turning off a system switch, which does not remove power from the system.
Deactivation may involve pulling and securing the circuit breaker and/or removing the
equipment. Deactivation of an inoperative system is not preventive maintenance as
described in part 43 appendix A. Regardless of the method of deactivation, a person
authorized to approve the aircraft for return to service under § 43.7 must make the
maintenance record entry required by § 43.9. No person may operate the aircraft without
the entry required by § 43.9.

Deactivation of any inoperative component can only be done by someone authorized to do maintenance, and they must make a log in the maintenance records to return the aircraft to service.
This is a change from the previous version of the AC mentioned in the other answer. Collaring a circuit breaker may still be acceptable, but it needs to be returned to service by a mechanic or someone else authorized to do maintenance on the aircraft.

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text:
How do virtual circuit breakers work?
In AW189, B787 and few other aircraft/helicopter, the physical breaker has been replaced by virtual breaker, which can be controlled by the MFD (Multi-Function Display).
I tried to google the answer, but I could find nothing about it: how does a virtual breaker physically work? I know how you control it, but what really happens when you press the breaker switch button on the MFD?

For details about electronic involved, look for thyristor in DC or triac in AC, as the power switching component is likely one of these latch devices. Those semiconductors are used for quite a long time for power switching (e.g. in once-pervasive strobe). The gate of those devices would be triggered by a simple current measurement (the voltage at the pins of a calibrated resistor) this measurement would replace the thermal effect in a thermal circuit breaker.

Could you please edit question and give some context (a link) or explanation of what "AW189" is.  "MFD" or "B787" is pretty obvious, but AW189 is not. What is it ?

@kebs I update the question.

The interesting part of the question would be: What happens if the virtual circuit-breaker for the MFD is triggered? ;-)

A virtual circuit-breaker is a relay (solid-state or otherwise) where the current going through it is monitored by a computer of some sort. When the computer sees too much current going through it, the computer trips the relay open. (Think of it as 'active CB' versus 'passive CB'.)
The engineer/pilot can access the computer to find out which CBs have been tripped by the system, and can try to get the system to close them again, just like resetting a real CB.
Advantage 1 - you can guarantee that the CB will trip at the target amperage, a real CB will often have a large error in its trip amperage.
Advantage 2 - real CBs have to be mounted where they're accessible to the crew, virtual CBs can be mounted anywhere that's convenient for the systems design engineer.


text:
Is there any electrical component on an aircraft which can never be turned off by a pilot?
Am I correct to assume that every electrical system on an aircraft can be manually switched OFF/ON by the Pilot OR is there indeed any system, on which the Pilot does not have the ability to switch OFF/ON ?

everything has a fuse that can be pulled

@ratchetfreak - not so, if the engine is running, it's computer can not be shut dow by either switch or fuse/breaker.

@SteveH you can still shut down the engine it is connected to

@ratchetfreak - not in flight, shuting the engine off simply send an electrical command to the computer to shut fuel flow off, but it does not shut the computer off. the engine will windmill and the computer will remain powered after shut down.

It should be possible to switch off the FADEC.

@shortstheory - It's not possible.

In the event of complete aircraft wide electrical failure (or some crazy pilot pulling every breaker in the aircraft), it should be comforting to know that the FADEC will continue to keep the engine running. for that it needs a power source independent of the aircraft power. The designers felt that there is no reason the crew needs to have control over this power source. Indeed, they probably have other things on their minds at the moment like the boss complaining that the internet is down ;-)

What about data recorders?

related: http://aviation.stackexchange.com/a/2268/1467

I'm not an aircraft person, but I would be surprised if the pilot could turn off the black box.

@user2635036 the pilot can turn off the black box, it has circuit breakers just like pretty much everything else electrical.

Well if you're looking for something to use in an argument, magnetos can have their circuit cut-off, but they're still always generating electricity when the engine is cycling.

You can always shoot yourself in the foot, if that's what you are asking.

FedEx Flight 705 is one example of a flight where the CVR was deliberately disabled, precisely by pulling its circuit breaker. (In that case, it was caught during the pre-flight and the circuit breaker reset.) I know I've seen at least one more example, a commercial passenger flight where the CVR and FDR circuit breakers tripped thereby robbing investigators of those recordings for the final part of the flight, but I don't recall which flight that was.

One example of a system that cannot be switched off by the pilot that is probably more common to all other airframes than just the one airframe I'm indicating.
Whenever electrical power is applied to a Gulfstream G550, whether it is powered by battery, generators, an apu, or some external power source, each engine's FADEC Electronic Engine Control (EEC) computer will be powered and the pilot can not switch them off.
The pilot could only indirectly turns them off by switching off all power sources.
However, when the engines are running, he can't even do that since there is an engine-driven generator dedicated to powering the EECs in the event of a total power loss.
In fact, while the engine is running or even windmilling above 35% rpm, the EEC's dedicated generator is supplying all power to the EEC and there is no switch or circuit protection that can be manipulated by the pilot to interrupt it.

On an R22 helicopter (I quote from the Pilot Operating Handbook):

Each tach, the governor, and the low rotor RPM warning horn are on separate circuits. Either the battery or the alternator can independently provide power to the tachs. A special circuit allows the battery to supply power to the tachs even if the master battery switch is off.

It is clear that the designers intend the the tachometers should work at all times, and there is no "off switch".
To Falstro's point, the procedure for an electrical fire in flight includes turning off the alternator and battery master, and this is the precise moment when I am pleased to find that the tachometers are still working, since the governor and RPM warning horn are not.
It is true that there is also a circuit breaker for the tachometers, but there is no procedure, emergency or otherwise, which demands for these to be pulled. The only time I have ever known these to be used is by instructors to simulate emergencies for training purposes.
The R22 is a pretty simple system and I don't know much about airliners but I imagine that it's impossible to access the flight data recorders in flight and I'm sure the designers try quite hard to make sure that they have power under all circumstances including catastrophic aircraft failure and hijack.

welcome to Aviation.SE! You have made a very nice first answer, to which I have made a few simple spelling edits and put your quoted paragraph into a quote block.

The pilot is generally given ultimate control over everything in the plane, when it comes down to it. Just like your car, they have to be able to shut everything down at the end of a flight. Take a look at this 737 classic overhead panel:

Original: http://www.angelfire.com/il2/aphs/73oh2.jpg
You will notice all important switches have an "OFF" position. The black and red switch covers protect the main power connection switches, which you generally want to be "ON" during the flight. The BAT switch at the top connects the battery to the DC electrical bus. The switches below that control the switching of standby power, and the connection to the engine generators. At the bottom are the controls to transfer between different power sources.
Of course there are many other switches for different systems. Which systems have dedicated switches will depend on the make and model of the aircraft. As others have commented, some low level systems like engine control may be absolutely necessary for some things to function, and will not be easy to switch off.
As ratchet freak commented, they also have access to the circuit breakers. Here is another picture from the 737, behind the first officer's seat. The pilot's handbook has instructions for what breakers should be switched off in what circumstances. Outside of those instructions, the effect on other systems of pulling one breaker could have unexpected consequences. The circuit breakers can be used to switch off equipment as a last resort.

In the name of safety, I don't agree completely with the idea of circuit protection being a responsible part of this answer. The pilot should not feel free to arbitrarilily make the decision to pull a CB. He has a reference handbook that tells him what CBs to pull in what situations and to go outside of that can make a dangerous situation. But I conceed perhaps a dire situation may warrant it. Anyways, because of this, there are many systems that can not be directly switched off and have no guidence for crew to deactivate by circuit protection.

The black and red switch covers protect the main power connection switches, which you generally want to be "ON" during the flight It not only protects them, but also makes them quick and easy to find; I don't know much about airliners, but in smaller aircraft, the first instinctive reaction to smoke in the cockpit would be to go for the electrical master switch.

This answer sort of depends on which system and which airplane. There are many circuit breakers outside of the cockpit that the crew cannot access.

@falstro What would be the procedure following? I would guess something like switch everything electrical off, then enable the master, and enable systems 1 by 1 as a diagnostic?

The difference between a car and a plane is that if something goes wrong in your car, you can easily pull over and, in the worst case, get out of the car and run away. The pilot of an aircraft doesn't really have that option, so if a device or circuit is malfunctioning, it might not be enough to be able to shut it off after the end of the flight; rather, they might need to shut it off now and continue flying the plane to a suitable landing spot.

The Indonesia Air Asia 8501 crash is an unfortunate example of why certain circuit breakers should not be reset in flight.


text:
Airliner sharp wing tip vs elleptical
Now that most airliner manufacturers are using composite materials for wings, making compound curves is much easier to produce, why aren't we seeing elliptical wings, outer panels or wing tips?
Aviation textbooks say the elliptical wing is the most aerodynamically efficient. If so, why aren't manufacturers using this to their advantage for better fuel efficiency, speed or range?
Also, what would be the approximate speed, fuel or range benefits of using an elliptical wing or wing tip?

Airliners have some boundary conditions which make elliptical wings less preferable.

Wing sweep. A swept, elliptical wing suffers lift loss at its center and will easily see its tips stall first at low speed. Both can be avoided by de-sweeping the wing at the center and giving it a nonzero chord at the tip.
Structural mass. Scaling laws tell us that the structure will become proportionally heavier as airplane size goes up. In order to built lighter wings, a non-elliptical planform with more lift and chord at the center and less towards the tips will built lighter, thus avoiding lift and, consequently, induced drag.
Transsonic aerodynamics. In order to keep wing isobars aligned with local chord lines, camber, incidence and the relative chord of maximum thickness must be finely tailored over span. The result is a rather unintuitive chord and incidence distribution but gives an overall optimum of maximum lift at minimal surface area and structural mass for flight in the troposphere at Mach 0.78 to 0.85.

Sorry, but there are no range benefits or fuel savings to expect from an elliptical wing. Speed is limited by Mach effects and planform will not affect this. For an airliner with an elliptical wing you need to go back to designs like De Havilland's Dragon Rapide. The next design, the Dove, changed to a trapezoidal wing.

An elliptical lift distribution is optimal in terms of induced drag.  It can be achieved by an elliptical planform (with a straight span and un-cambered airfoils and no twist), or by twisting a more complex wing.
An elliptical lift distribution is not optimal in terms of wing weight.  It may also not be optimal in terms of handling qualities, tip stall, flutter, aileron reversal, etc.
Modern composite wings also flex a tremendous amount in flight some of this can be planned for, but you still need to design a wing that operates well at off-design conditions, not just at a narrow design point.
The theoretical elliptical wing is un-swept.  To fly at transonic speeds of an airliner, you must have a swept wing.
So, with all that, a traditional elliptical wing is not optimal for an airliner.  There are many good reasons not to use one.
That said, composites would allow more swoopy curves on a wing -- while wings still appear to be mostly straight.  Even if they aren't elliptical, why not use more compound curves?
There are still good reasons to prefer straight lines.  Airliner wings have control surfaces and high lift devices all over them -- slats, flaps, ailerons, and spoilers.  All of these are mechanisms that need to operate without binding.  To do so, it is best to make the hinge line straight -- or to design with aligned tracks and linkages.
Even with a straight hinge line, the 787 breaks the control surfaces into many segments -- to accommodate the flex of the wing.


text:
Do fuselage rip/crack/damage sensors exist?
Are there sensors that can detect a rip, crack or damage in the fuselage skin by not using air pressure differential on aircraft?

Something down that alley discussing control of damaged aircraft. I don't think exact damage location knowledge is a priority though since it will be hard to estimate and evaluate; that can be done on the ground. In theory I guess you could go sticking strain gauges or optical fibre sensors on the fuselage, but it will be hard to do it effectively. Aircraft do record forces that can give a good prediction if problems are to be expected.

I'm guessing you are asking whether it can be done in flight. What do you mean by "not using air pressure differential"?

Actually should say air pressure loss. You're right..

Ever heard oh [Aloha 243][http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243]? It does not exist then, it does not exist now. Proper maintenance and preflight helps detect these damages before they fail in flight.

@vasin1987 Aloha 243 happened in 1988. Plenty of things that didn't exist in 1988 do exist now (the world-wide web, for example) so that's not really a very good example.

Such sensors certainly exist. The sensor is as simple as can be: usually a piece of wire. The bonding of the wire to the skin or structural elements dictates what kinds of damage it's sensitive to.
The "wire" can be in the form of a thin metallization on a thin non-conductive substrate that's intimately glued to the skin - such gives the most sensitivity to cracks, and is a crack detector.
If you wish to detect whether the rocket is in the process of breaking up, you simply need to run a thin, insulated wire along a longitudinal structural member, such as a tank, and attach it in a few places, but certainly rigidly at the ends. The input is used sometimes for automatic flight termination.
The major problem with pressure differential sensors is that you really need a big hole to overwhelm the pressurization systems on turbine-powered transports. A pencil-sized or even coin-sized hole is noisy but essentially irrelevant, and no, you won't get sucked out through it.
Why is it so? For cabin ventilation to work to begin with, there must be an outflow valve. Such valves, in steady state, have openings way larger than a coin. The more unplanned-for holes you add to the pressurized section, the smaller the outflow valve's opening will be. Only when the outflow valve completely closes and the pressure is still insufficient, you've got a problem...
So yes, the outflow valve's position in steady state can be used as a measure of air leakage elsewhere.

It's not typically used for automatic flight termination in airplanes, though. :) We tend to avoid putting automatic flight termination systems in airliners. Pax don't like them... pilots don't either.


text:
What are the benefits of an FAA over an ICAO license, and vice versa?
I will study for commercial pilot in May but I don´t know where I should study. I have 2 really good schools, one is in Canada (ICAO) the other one is in Florida (FAA),
What are the advantages and/or differences of both licenses when it comes to a career in international aviation and what potential differences may it have in career and training paths.
If we only talk about license not about schools.

Hi.  Welcome to Aviation StackExchange. prestige - this might be a translation error.  Please could you try different words to describe exactly what you mean.

I have edited this to drive more factual and less opinionated answers as per the guidelines of this exchange.

There's not really such a thing as an 'ICAO licence' for pilots. You will get either a Canadian or a US licence, and if you then get a job in Dubai you'll need to convert the licence to a UAE one. The linked question should clear it up a bit. FWIW, the US licences are a bit more annoying to convert in my experience, as the US doesn't comply with the ICAO English language requirements, so US pilots often have to take another English test. Go figure!


text:
How has weight been reduced in commercial aircraft design and how will this trend continue?
Reducing weight of aircraft has become a priority as the aviation industry seeks greater fuel efficiency and commits to strict emissions targets. How much weight has been cut from single and twin aisle aircraft in last 10 yrs and where? How much further can OEM's and suppliers cut weight in the next 10 years and in what components/equipment/furnishings will the greatest savings be made? I am looking for some fun and relevant facts on this subject for an article I am writing.

Losing flight engineers saved about 300 kgs.  50 kgs for the seat, 50 kgs for the flight bag, the rest for the wetware talking ballast, coffee and meals ;)

Perhaps the biggest weight reduction in aircraft across the board has been the use of composite materials over aluminum. This extends from small GA aircraft all the way up through airliners like Boeing Dreamliner. In the retro fit game, some older planes have been certified to take newer composite propellers. Even small savings matter according to this article

LED modules give on average a 40% weight saving over the incandescent
  modules they are replacing, which improves fuel economy

Glass cockpits and modern computerized avionics tend to weigh less and offer more than their earlier steam gauge counter parts. Even in the space shuttle.

The system also provides greater backup capability, weighs less and
  uses less power than the original design.

Another thing that has made a fair dent in take off weight is how fuel efficient engines have gotten. While there is a practical limit to this as engines have gotten more efficient the need to carry fuel has dropped or planes can go farther on the same fuel. In a similar vein as weather planning/predicting has gotten better fuel loads (while still in compliance with FAA reserve regulations) may be slightly lighter as they can now be more precisely calculated.
The one thing that has gotten heaver is the passengers...

This, of course, only is feasible as manufacturers manage to wring the additional difficulties and added costs of manufacturing parts from composites (which have been VERY problematic in the industry - see the 787 and the ill-fated Lear 85).

Besides the use of composite materials, another weight reduction technique has come from creating lighter weight electrical wiring and cables used throughout the plane and by reducing the amount of wire needed overall.
This has been addressed in several different manners.

Reduce the overall weight of wire and cable per foot by developing wire that uses light weight materials or that can withstand higher temperatures so that less insulator or conductor is needed. The Shrinking Size of the Cable
Design electrical, flight, and data systems that require less wiring in the first place. By reducing the amount of wire needed, the overall weight can be reduced significantly. One example is Avionics Full-Duplex (AFDX), which is used to replace ARINC 429 data networks on large aircraft. AFDX requires less wiring than ARINC 429, thereby reducing the overall wire weight.

I know what I'll say is not exactly what was asked, but I found very interesting in the same way :-)
If you look at GE jet engine's technology you will probably get impressed with its details. Especially the carbon fiber composite material is something that allows GE to make fan blades longer and thinner. Look at: http://www.gereports.com/the-art-of-engineering-the-worlds-largest-jet-engine-shows-off-composite-curves/

I've tried to extract some relevant content, and here it's:

Nick Kray works as a consulting engineer for composite design at GE
Aviation. In the 1990s, he was part of a GE high-stakes gambit to make
the front fan of its largest jet engine from epoxy and carbon
fibers.
The blades from the material, called carbon-fiber
composite, allowed GE’s aerospace engineers to design the GE90, still
the world’s largest and most powerful jet engine. It’s also GE
Aviation’s most profitable machine. “Our competitors make jet engine
fans from titanium and steel and even some of our own people weren’t
initially so hot about using composites,” Kray says. “Nobody had tried
this before.”
The material allowed GE engineers to design blades
that result in lighter and more efficient engines, allowing airlines
to save fuel by shedding precious pounds. Now Kray and his team
are busy building the future. They are working on a fourth generation
of the blade for the GE9X, GE’s largest engine yet, designed
exclusively for Boeing’s next-generation wide-body jet, the 777X.
The blades will feature several new components, Kray says. They will
use stiffer carbon fibers so GE can make them longer and thinner.
Their trailing edge will be made from a special structural glass fiber
composite that can better absorb impact energy.
Where the GE90 has
22 blades and the GEnx holds 18, the GE9X will have only 16, even
though it is the largest of the three.  Besides making the engine
lighter, the fewer and thinner blades will also spin faster.

GE is an OEM/supplier, so I think reducing weight in an engine applies. Assuming, of course, that the CF blades are lighter than traditional ones.

I think now it's better @J.Hougaard. What di

What do you think?

The other answers to this question are all true, good and relevant. However, there are two important things that have not been mentioned so far.
One is that aircraft parts, especially critical parts such as wing spars, are less and less over-engineered than they used to be.
In the absence of more precise knowledge about the load-bearing, bending and other limits of important structures, aeronautical engineers would calculate for very large safety margins, making those structures much stronger and resilient (and heavier) than they would ever need to be.
Better understanding of the limits - knowing more about how and when structures will fail - has allowed those safety margins to be reduced safely, making the structures lighter.
Note that reducing safety margins doesn't necessarily mean making something less safe. A huge safety margin in wing design that allows a wing to withstand forces that would damage other critical parts or kill every person on board doesn't actually mean improved safety. Past a certain point, it just means increased weight.
And, reducing unnecessary weight allows more scope for improvements in other areas that in turn could improve safety.
Secondly, new computer-aided design techniques have begun to come up with and explore designs that no human could or would, based on mathematical modelling and simulations that produce unexpected results.
For example, some techniques have produced designs in which material is cut away (from, say, a cylinder or beam) in patterns that appear to be random. For the same strength/rigidity/resistance to failure, the part can be made very substantially lighter.
Both of these trends will doubtless continue, and because they can apply equally to well to structures like wings and tray-tables, they're going to have implications for all aspects of the design and engineering of aircraft.

There are several measures that I can think of. I'm sure some of them only save weight as a side-effect, rather than that being the main driver for the change.

Switch to composite materials. This has definitely been driven by weight-saving. Building the wings and fuselage of lighter materials makes everything cheaper.
New seat designs. There are new slimmer seats, mostly in short-haul aircraft. The seats are much smaller, and quite a bit lighter. This is probably driven more by the ability to add more rows and still maintain decent pitch, but is also a weight saving measure.
Wireless and BYOD entertainment systems. Again, mostly short-haul. I've had aircraft with no entertainment at all, or with bring-your-own-device wifi systems. These save the weight of the devices, and especially of the wiring.
Toilet design. This is mostly for the weight of the systems themselves these days, though it also saves the weight of the water. I think a lot of the changes to these designs were actually safety and comfort related, though.

It is possible, that all of these changes are outdone by change in engine design and aerodynamics (winglets/sharklets), which reduce drag and save the weight of carrying more fuel  on board, though I don't have data.


text:
Would titanium be a better option for aircraft skin if cost were not a factor?
If price was not an issue, would titanium be a better option to use as the skin of airliners as opposed to aluminum?

Specific strength is not always the only consideration. Strength over weight is the factor here, if a material has a third of the weight but also a third of the strength of another material, they have the same specific strength. Aluminium, titanium and steel are the metal alloys for aircraft construction, and they have roughly the same specific strength. There is a large variety within alloys of course, 2024 aluminium alloy has a yield strength of 324 MPa and is over 30 times stronger than pure aluminium.
Image source
A wing in flight is bent upwards by the aerodynamic lift forces, and this bending moment is resisted by the upper and lower skin in the wing. The lower skin is loaded in tension, the upper skin in compression. With a tension loaded construction, we're only concerned with the cross sectional area: how much force does it see, in order to get to the yield strength. With a compression loaded construction however, the failure mechanism is not yield strength but buckling.
In order to prevent buckling, we need a larger cross section than what would be necessary for pure yield strength, and this cross section is a function of the elasticity modulus, not the yield strength. So now the ratio of weight over elasticity sets the dimension. For compression, aluminium has the best ratio of the three metals mentioned above.

And this is not even considering whether various composites might have even better characteristics. E.g. the Boeing 787 that's about 50% composite.

Yes - low cost and highest strength can be aligned with the direction of the  highest stress.

even assuming price would not be an issue, formability certainly would be. Ti is much harder to bend, drill and cut than Al so building a plane out of cheap Ti would significantly increase its manufacturing costs over Al. Ti retains its strength at high temperatures far better than Al but in subsonic aircraft like passenger planes, airframe heating is not a design limiter that would drive a choice between selecting Ti or Al.


text:
Is there a comprehensive source for aircraft specification data for most of the aircraft in the world?
I'm aware that I can look at the Wikipedia entry for an aircraft and discover specifications like the MTOW, length, cruise speed, etc. However, I was wondering whether there is a (preferably free) source of all this data combined for most aircraft. Does such a data source exist?

Most? From Wright's flyers up? 2 seaters to A380? Russians built as well as western built and china built? Then there is too many. Can you scope down a bit of what you want?

By MTOW

No scope. But I appreciate a database would be incomplete.

FWIW, there are a few weight & balance manuals for the 747 at http://terryliittschwager.com/wbmanuals.php. These manuals were supplied to me in connection with weight & balance software development and with no mention of confidentiality or proprietary.  Interestingly, the latest manual supplied to me has in prominent blue letters the word "proprietary", so I've not put that manual up.

What you want all planes specs for? Some research or personal use? If you list which plane you want to know maybe someone may have them handy but no one will have specs of most planes at hand.

I want to compare aircraft by certain specifications as an exploratory exercise. I appreciate that any list will be incomplete, but a rudimentary search hasn't come up with any. I'm interested whether someone has compiled a database already. I'm not interested in the specifications of a particular aircraft or a particular specification of all aircraft.

Try this : http://www.aircraftbluebook.com/Tools/ABB/ShowSpecifications.do mostly small aircraft though

Thanks!! That's exactly the sort of data source I'm after. I'd still like to know if there are any more comprehensive data sources -- including high-capacity aircraft.

Nah. If you tell us EXACTLY what parameter you are after and what class of aircraft you are looking for that might be easier for all of us. As you can see we have all the range of professional here I would appreciate if you can specify what you want so we can help you.

This question asks for a single source, it is not "too broad".

I'm voting to close this question as off-topic because link-only answers are not acceptable on Stack Exchange and they are the only possible answers to this question.

You can find a lot of information about aircraft in their Type Certificate Data Sheet that is available from the FAA or other certificating authorities. The FAA's is located here. It takes a bit of time to find certain things, and other things aren't included like range, or sometimes dimensions.

This isn't really what I'm after. These are multiple files, whereas I'd like a single, machine-readable source. If you like, a file where every row is an aircraft and every column is a spec.

@Hugh It is a single machine-readable source. Your computer is most likely capable of retreiving files from the internet in an automated manner.

I disagree. But thank you: this is a comprehensive, if unwieldy, source of data.

Jane's All The World's Aircraft
It's not free (Amazon lists it for between \$500 and \$1,000 USD) and it's not machine-readable, but yes, the data is out there.

here is one for a few large transport aircarriers https://booksite.elsevier.com/9780340741528/appendices/data-a/default.htm

Yes there it. The EUROCONTROL Aircraft Performance Database


text:
Is it common for carriers to have separate certification for analog versus glass cockpits?
Is it common for carriers to have separate certification for Analog cockpits versus Glass (digital) cockpits?

Pilot type ratings are usually same for both versions (analog and digital cockpits) of the same aircraft as far as regulations are concerned. For transition to glass cockpit, you may have to take a Electronic Flight Instrumentation Course (or differences training course).
In some cases of transition (for example, EASA  ATR 42/72 (glass cockpit, non
ECL) to any other ATR 42/72 'classic'), a full type rating training course is required; however, the type rating is still the same.
For example, see the FAA and EASA type rating lists.


text:
Why does the Grumman Bat feature inverted winglets?
I've been fiddling around with trying to make a blended wing body aircraft. While modelling, I heard from someone that the design reminded them of the Swift Killer Bee (now Grumman Bat). One distinct feature that I noticed from the design of the Bat is their use of an inverted winglet. Does anyone know why the design features an inverted winglet rather than a conventional one? The more recent designs of the UAV also features a conventional winglet in addition to the inverted one. Are there any benefits for this design?

I would also like to know if anyone has an idea on how the UAV handles landing? Is the inverted winglet capable of supporting the stresses experienced during landing? I can only find that the Bat comes with an optional landing gear while normally it is supposed to be caught by a net.

FWIW those aren't winglets. They're vertical stabilizers also called "fins" on flying wings. They provide a different function from winglets though sometimes they CAN be designed to also function as winglets (see the Prandtl flying wing)

Without knowing what was going on in the minds of the designers it's difficult to tell exactly, however there are a few reasons as to why the vertical stabilizers are featured both above and below the wing.
In the sUAS market there are a huge amount of products which offer very similar capabilities. It is quite possible that the stabilizer design of above and below rather than in a single direction is to differentiate the product in a crowded market. This is similar to business jet design wingtip design - while it is known drag may be reduced the devices offer aesthetic appeal and different designs stand out to buyers.
The design also allows for the same surface area with a smaller packing footprint. Due to the bulbous shape of the fuselage the potential package size of the bat is smaller with the stabilizers as designed rather than simply above or below.
The stabilizer design is symmetric around the horizontal plane, meaning that in a side gust condition the UAS will feature less tendencety to roll and will instead simply be 'shifted'.
Again in a side gust condition the balancing of forces above and below the attachment point cancels out the moments and therefore may allow for a less strong joint (weight savings).

Winglets in general increase aircraft performance by decreasing induced drag caused by air spilling over the wing tips from the bottom to the top of the wing (sorry, I forgot the technical term). The winglets reduce this spillover.
Since the aircraft is a pusher, there is the distinct possibility of striking the prop on the ground on takeoff or landing. The inverted winglets will act as a guard to prevent this (as the inverted V-tail design on the predator drone protects its pusher prop from the same such prop-strike).


text:
What is an acceptable phugoid oscillation duration?
What is an acceptable settling time for the phugoid mode in a large aircraft?
As a more specific question, what is the settling time when the aircraft (say, a B747 or an A320) switches from Climb to Cruise phase at the top-of-climb?

Here is the response of a 180 passenger high-subsonic twin-jet simulator: the first period takes about 87 seconds.
Conditions:

From the trimmed position, excite the phugoid mode by applying longitudinal control in one direction in order to change airspeed by approximately 10 kts, then release.

Initial conditions:

COG 0.2205 MAC
Pressure altitude 35,000 ft
Engine N1 rotor speed 87.47% (both engines)
Calibrated Airspeed 264.54 kts
Gross weight 66,360 kg

EDIT
To answer the additional question of when the resulting motion amplitude would be below 0.1 deg, I made a kind of a digital simulation according to this block diagram:

and then tuned spring stiffness C, inverse inertia 1/M, and damping constant K until the resulting response overlaid the original aircraft one. The resulting motion gets to a max. amplitude of 0.1 deg in about 1,600 sec - 27 minutes.

What do you think is the approx. settling time, maybe when the amplitude goes below 0.1 deg?

The peaks setlle down over time, a lime through the peaks looks like a first order response. I'll see if I can make a ROM on < 0.1 deg later in the day.

Normally this is in the tens of seconds.
An approximation for the undamped phygoid frequency is: $$\omega_P = \frac{2\cdot g}{v}\cdot \left(1-\frac{c_{mv}\cdot c_{L\alpha}}{c_L\cdot c_{m\alpha}}\right)$$
Nomenclature:
$g\;\;\;\;\;$Gravitational acceleration
$v\;\;\;\;\;$Flight speed
$c_{mv}\;\;$Speed stability (change in pitch moment over speed)
$c_{L\alpha}\;\;$Lift curve slope
$c_{L}\;\;\;$Lift coefficient
$c_{m\alpha}\;$Static stability (change in pitch moment over angle of attack)

Can you comment on the oscillation just after the top-of-climb? Is it phugoidal in nature? And it follows the same frequency formulae as is in the answer?

@AnandS: I fail to see a difference between infinitesimally small oscillations and straight flight. Damping ensures that those oscillations die down (except in some gliders), so yes, a well-designed airplane does fly perfectly straight if we neglect that it follows the curvature of the earth.

@Raj: Those oscillations (if they occur at all) are short-period oscillations.

Let's first address OP's first question:

What is an acceptable settling time for the phugoid mode in a large aircraft?

Phugoid is a hands-off (i.e. pilot and autopilot off) rigid-body mode, with or without Stability Augmentation System. MIL-STD-1797B specifies a damping ratio of at least 0.04 for Level 1 handling quality. The typical phugoid period is around 90 seconds. This converts to a settling time (within 2% of trim speed) of around 24 minutes. This is so long which makes settling time itself not a particularly good measure for phugoid.
Now onto,

What is the settling time when the aircraft (say, a B747 or an A320) switches from Climb to Cruise phase at the top-of-climb?

Leveling off from climb is not hands-off. A well-tuned autopilot should be deadbeat with minimal overshoot and oscillation. The time scale is also vastly different; we are talking about settling time on the order of a seconds (not half an hour). And this not phugoid.

Mil-F-8785C is rescinded, however exact same requirements carried over to new milspec. So your answer still holds true. Strangely enough, civilian counterpart Part 25 section 25.181 is not even bothering to give any requirements for phugoid. I think that’s because phugoid is a nuisance mode.

@Kolom Is MIL-STD-1797A the replacement?

1797B is the new one

This should be the accepted answer.


text:
Has anybody done gliding experiments with exact replicas of the birds or other real world flying creatures?
The most obvious way to verify if birds are stable or not in at least gliding flight would be to make the exact rigid model of the bird (including weight distribution) and launch it into flight. The simplest approach that even early aircraft inventors may have tried is to use a frozen real bird.
Are there any references about anybody trying such experiments? Pure logical or mathematical analysis is outside the topic of this question.

A number of experiments and papers by Robert Hoey (from the early 1990s on) have used models of flat-winged soaring birds such as ravens.
One of his papers is referenced in this broad review: Lentink & Biewener 2010
One of his earlier papers is copied here: Hoey, 1992, American Institute of Aeronautics & Astronautics
The topics include some discussion of low static stability of the model and the lack of vertical surfaces in real birds, as well as unknowns such as wing profile.

Yes, this is that is required.

The closest I know of are several attempts to build models of pterosaurs.
Since the head with its long beak is ahead of their center of gravity, the configuration is statically unstable in yaw and needs continuous control corrections. On the other hand, this instability provides very quick responses. This YouTube movie contains footage of a Pterodactyl model made by Paul MacCready in flight. Note that Paul had attached a conventional tail for the first flights to statically stabilize the model in pitch and yaw. Later in the flights this tail was removed. The small skin area between the legs was most likely not sufficient for pitch stabilization, and small changes in wing sweep were used to stabilize the animals in pitch, just like modern birds still do.
Another branch, the Rhamphorhynchoidea, had a tail which provided additional lateral and longitudinal control. Palaeobiologists are still arguing, however, whether the tip of the tail was oriented horizontally or vertically in flight.

LOL! For some reason I read "McCready" as "McCartney."

just like modern birds do You mean, ancient birds did it another way?

Ancient birds I assume Archeopteryx.

I don't know about birds but researchers have been working for a long time to replicate insect flight. This article talks about how 3D printing has made this much easier. It has a video of a working replica hovering. They call it an ornithopter. Technically "ornitho-" means "bird," but I guess entomopter didn't have the right ring to it.
I remember reading something back in the 80's about spy agencies trying to develop a "fly on the wall" cyber-insect that could get in through air ducts or other tight spaces.

This is interesting, and I also upvote, but that from the picture machine seems not a realistic copy of any known species and would be extremely huge as for a fly. Butterflies do glide but I do not know if a fly could.

I don't think there are a lot of gliding insects. Most that I can think of use rapid flapping motion. Since you specifically asked about gliding my answer doesn't really apply. Just thought it worth putting out there since you mentioned "real world flying creatures."

The word ornithopter (meaning a flying machine with flapping wings) is more than 100 years old--long predating Dune. See http://www.dictionary.com/browse/ornithopter (particularly several of the early examples of usage).

@DavidK Cool. Always figured Frank Herbert made that word up. I still think entomopter would be more accurate. Ornithopter is made from the classical Greek words for "bird" and "wing." Whereas "entomo-" is from the Greek word for "insect." I did notice I misspelled, tho.

@mins Now I'm SURE the NSA is watching me! Next time I swat a fly I'm going to check for a camera!

I agree, entomopter would make a lot of sense for a machine that is supposed to emulate an insect. As it turns out, another group actually does call their machines entomopters: http://www.gtri.gatech.edu/casestudy/flying-mars


text:
With wind, what will a perfectly circular flight path relative to the surrounding air look like from the ground?
Let's suppose that an airplane flying at a uniform airspeed follows a perfect, level, closed circular trajectory with respect to the mass of air. Now, seen from the ground, and in the presence of a constant wind, will no longer be a circumference. What would it be...?

It will be this :
http://www.aeroexperiments.org/images/grndtrck.gif
Pilot's of slow-flying aircraft like hang gliders and paragliders see tracks like these all the time on their GPS displays, as they circle in a thermal updraft in the presence of strong wind.
Depending on the strength of the wind, the "curliques" may become so stretched out that they don't form enclosed loops at all, just "points", i.e. a "scalloped" line.  This happens when the windspeed is greater than the airspeed. That means that the aircraft never makes any progress in the upwind direction at any point in the circle.
Related:
http://www.aeroexperiments.org/nocss.shtml
http://www.aeroexperiments.org/introcircles.shtml
http://www.aeroexperiments.org/circles.shtml

But if the 'air curve' is a closed one, the 'ground curve' has to be closed too, since a transformation of the reference system cannot 'open' a closed curve...

@xxavier as shown by the first image, the ground track is indeed open. there is no reason why it should be closed. your "if the 'air curve' is a closed one, the 'ground curve' has to be closed too" is not correct.

@quietflyer I don't know where your "curliques" names comes from, but the proper name is Prolate Cycloid http://mathworld.wolfram.com/ProlateCycloid.html or Trochoid http://mathworld.wolfram.com/Trochoid.html

@Federico Yes, you are right. If I have two plates in relative, parallel relative motion, and I trace a closed curve on one, its 'image' on the other will not be closed. My mistake came from imagining the first curve already existing, and not in the process of being traced...

Thanks for the terminology note.  I just decided to call the shape a "curlique" because I couldn't think of anything better.   It is not really the right word and apparently I am mis-spelling it to boot.  https://en.wikipedia.org/wiki/Curlicue

Now here's the fun part-- what precisely is the source of the force component that causes the acceleration component that makes the magnitude of the groundspeed vary?  Someone could ask that as a question, if they don't see the answer already.

@quiet flyer. Not such a force exists because that acceleration is related to the image of the aircraft, that has no mass. The only mass is that of the real aircraft, that flies in the air. Its image, moving on the ground, has no mass...

The real aircraft has a real airspeed and a real groundpeed and if the real groundspeed is changing in magnitude, a real force component, acting parallel to the groundspeed vector, is causing it.

Forces are only involved in the aircraft-air relationship. The aircraft has no physical relation with the ground while in flight, and hence, no forces ground-aircraft exist at all...

Never mind the chat, I need to call it a night.

Actually the answer to acceleration question is given in one of the url's attached to the answer above.  The "introcircles" one

In the particular case where the airspeed is equal to the wind speed, the curve traced is a cycloid, which is the same path traced by a point on the tread of a bicycle wheel rolling along the ground without slipping.


text:
Was the Buran space shuttle capable of doing a go-around?
In several places I've seen reference to the Buran automated landing where it supposedly did a go-around and came in for a second landing attempt.  I was under the impression that the Buran was unpowered and therefore didn't have any ability to fire engines to regain speed (and therefore altitude) for another landing attempt.  Is this go-around an urban legend, or am I mistaken about what Buran was capable of doing on landing?

Related: Was the unmanned flight and landing of the Buran spacecraft in 1988 exceptional?

The landings of gliding craft like the 'Buran' or the 'Shuttle' are probably fully computerized, so that 'go-around' is probably false. However, and at least in theory, the glide path may be intercepted by the aircraft at different altitudes, if it's also true that the 'Buran' or the 'shuttle' are notoriously bad gliders, and that the altitude lost between two interception attempts would be very large... Hence, it's better to do it right the first time...

It had two approaches, that isn't to say that it was a go-around, two different things. You can abort an approach and attempt another with a respectable amount of altitude and distance from the landing site.

Unlike Space Shuttle, Buran could cary Saturn AL-31 jet engines. The testing version had four such engines and used to take off and then fly under own power during the tests. Hence it so should have been capable of go-around. The powered landing increased the safety of Buran in comparison to Space Shuttle (source).
Here it is possible to find the description of the jet engine system of Buran. This source names the engines as АЛ-31Ф (Su-33 has the same). It says two engines were foreseen for the space going version, and this would have not been sufficient to take off under own power. Each engine had the maximal thrust of 7770 kgf (afterburners were removed). The two extra engines of the take of - capable prototype were more powerful (12500 kgf).
The source also says jet that engines were not installed during the only space flight 15 November 1988 while there were non-functional engine controls in the cockpit. The engines were not ready yet.
Hence most likely the sources OP references talk about some go-around of the mentioned four - engine testing prototype, not about the go-around of the Buran returning from space.
The space going version of the Buran space shuttle was not capable of go around maneuver.

Your source shows pictures of the Buran during its first (and only) unmanned space flight and does not have the jet engines installed. The jet engines were never used for for a rocket launched Buran, only the in-atmosphere test vehicle. The OP is asking about the automatic landing during its only space flight.

I was told the Buran did an unpowered go-around due to possessing too much energy to land.


text:
Can the center of gravity ever fall outside the mean aerodynamic chord?
Can the aircraft center of gravity be outside the mean aerodynamic chord (i.e. %MAC being negative or more than 100%), at least theoretically?
I'm not an expert in aviation. All I've heard is the safe CG range on nearly all aircraft lies between 10 and 50% MAC.

Yes it can be. Whether the aircraft can fly with the centre of gravity outside this range is a different matter altogether.
When flying, the aircraft is balanced around the centre of gravity of the aircraft. Aerodynamic forces from the wing and the tail provide the balance, but up to a maximum. At some point the aerodynamic lift capacity of the tail is saturated.
In order to make sure that the balance can be achieved, the centre of aerodynamic force must be within certain limits of the centre of gravity. Or the other way around: CoG must be within certain limits of centre of lift, which is the better way to define it, making the CoG the variable.


text:
Where can I get statistics on landing incidents?
I sometimes have the feeling that Airbus with augmented control are more prone to landing incidents (hard landings and runway excursions) than other models of aircraft with conventional controls.
Is there anywhere I could find data to perhaps back see if this might be correct? Such as a list of all landing incidents in Europe or the United States?

avherald has a list of incidents, you can try looking through that (not all incidents are recorded)

@ratchetfreak that's the problem with avherald :/

Interesting question, my feeling is exactly the opposite. Not only would we need a list of landing incidents per aircraft type (manufacturer), we would also need to know the number of flights per aircraft type to correct for differences in usage.

If organisations like the NTSB don't publish this information, I guess it's possible no-one collects it, or not in a sufficiently consistent form that allows comparisons.

You'd have to do an enormous amount of data correction - winds, pilot experience, etc in order for this to be of any practical use, and I don't think that this is something that can be easily 'proven'

Even if you find the data you're looking for, you need to be very careful with the analysis. If you're not an expert in statistics, your answer is unlikely to be valid. It sounds very much that you have a pre-conceived notion of what the answer should be and that you're looking for data to support that hypothesis. It's very easy to massage statistics, consciously or unconsciously, to get the result you're looking for.

NASA collects statistics on aviation incidents through its Aviation Safety Reporting System, and the ASRS database is searchable online.  You can specify criteria such as the flight phase and aircraft type in your search.

To actually address your 'feeling' of course you would need to combine this with statistics such as the number of landings made by each kind of plane.

I ... feel ... [aircraft] with augmented control are more prone to landing incidents

The Airbus and Boeing 787 FBW with augmented control have some similarities.
This 2012 ICAS paper: "FLY-BY-WIRE AUGMENTED MANUAL CONTROL - BASIC DESIGN
CONSIDERATIONS" discusses Boeing and Airbus augmented control systems and its effect on piloting technique
See 4.2.

4.2 Piloting Technique for Disturbance Compensation
The influence of manual augmented control on the optimal
  piloting technique is significant. For conventionally
  controlled aircraft the task of stabilizing the
  aircraft on the desired flight path after an external disturbance
  is left to the pilot, but the pilot has no suitable
  indication of flight path deviation on his PFD. For
  that reason pilots are still used to react to a change of
  attitude angles. As the attitude angles are easy to derive
  either from the outside view or from the PFD that
  puts the main focus on the aircraft attitude control, the
  main control references for manual control are still the
  pitch angle for longitudinal control and the bank angle
  for lateral control.
However, the application of manual augmented control
  in modern FBW aircraft has influenced the distribution
  of tasks between the pilot and the flight control
  system with respect to the compensation for external
  disturbances.
For lateral motion control, Airbus uses a roll rate command
  and bank angle hold system which actively tries
  to keep the bank angle at zero after a lateral disturbance.
  The B777 FBW lateral control is basically a
  Direct Control mode without feedbacks and a roll attitude
  hold feature, so the airplane responds to a lateral
  disturbance like a conventionally controlled airplane.
  The B787 has an integrated lateral/directional control
  algorithm that provides basically roll rate command/
  roll attitude hold control, similar to the Airbus design.
  The latter type of designs provide more roll attitude
  stiffness and thus require less pilot compensation for
  external disturbances. For those control algorithms a
  strong reaction of the pilot in response to rotational
  rates or attitude angle changes might lead to adverse
  effects due to a significant interaction between pilot
  and the flight control algorithm.
In 2010 DLR performed a study on a Airbus A330
  Full Flight Simulator, where the aircraft was disturbed
  by a simulated wake vortex encounter while the pilots
  revealed
  that numerous pilots aggravated the impact of
  the wake vortex on the aircraft response by generating
  to strong and to abrupt inputs on the stick, although
  they were informed that a wake encounter could occur
  [23]. This was especially observed for pilots with
  little experience on FBW aircraft. The final report
  recommends that the pilots should react to the lateral
  and vertical flight path deviation with extremely
  smooth inputs on the stick rather than trying to overcompensate
  for the change in attitude angles [23].
  These recommendations are similar to those given
  by Airbus for manual control during wake-vortex encounters
  [21].
Hence, the piloting technique should change from a
  continuous compensatory control to an intermittent
  control strategy, because the short-term control task
  of continuous pitch and roll attitude stabilization is
  performed by the control algorithm. In many current
  airline training programs no emphasis is put on the
  adaptation of the piloting technique that is needed to
  safely operate a FBW aircraft with manual augmented
  control in case of external disturbances. It seems that
  in most cases the pilot still uses pitch angle as his primary
  control variable. During the simulator study performed
  by Gautrey several pilots commented on the
  desirability of a flight path vector display, especially
  for the unconventional systems without speed stability,
  because this is the parameter which they are ultimately
  trying to control [12]. For the majority of existing
  Airbus FBW aircraft the flight path vector symbol
  is only available in the PFD, if certain autopilot
  modes are triggered. However, flight path information
  is displayed in the head-up display which can be
  optionally ordered for every Airbus FBW aircraft.


text:
What could cause GE and Rolls-Royce turbofan engines to sound different?
I've often as a passenger noticed the difference of the sound coming from different airliners. I believe the two manufacturers I have compared are Rolls-Royce and GE, where the latter's engines sound more "electric" (funny enough) and the former's more distorted and noisy.
Do these observations make sense? And what could possibly create such a difference i sound? I was in the belief that engine design of turbofan engines is somewhat established and detailed.

The GE90 engines have a higher bypass ratio than the RR Trent.
The majority of engine noise, especially the "ripping, howling, typical jet noise" comes from the engine core.
The higher the bypass ratio, the less air passing through the core for any given thrust and the more cold air meets the core exhaust at the back reducing the shear
Jet engines with no bypass ratio and deep intakes produce a wonderful sound

Don't forget the difference in the number of shafts. The Trent should emit at more frequencies, because its parts rotate at three different speeds.

@PeterKämpf True, and the Trent has a contra-rotating spool, but I don't know what effect that has on noise.

Intake resonance - particularly evident on the Avro Vulcan.  It is a sound that speakers can do no justice to.

I had the pleasure of watching a Wing Commander get grounded (all pre-planned of course) on his retirement flight.  Took off, kept the howl going all the way round the circuit, then stood her on the tailpipes directly above the squadron office. Broke a few coffee cups and some windows.  Never to be forgotten :)


text:
Is there a comprehensive list of IFR terms with definitions?
Is there a comprehensive list of IFR terms with definitions easily available?  I am trying to make some flash cards for study

If you have a spare 99 cents you can buy my app and save yourself the trouble of making flash cards. I’ve taken all of the glossaries from FAA Books and combined them into an app where you can quiz yourself or look up specific terms and acronyms. One of the categories is Instrument Terms. https://itunes.apple.com/us/app/faa-glossaries/id454061073?at=10l9Em

@JScarry That looks like an answer. Why not turn it into one?

The FAA's Instrument Flying Handbook would be a good start, in particular the glossary.

FAA-H-8083-16B  Instrument Procedures Handbook also has an appendix of acronyms and a glossary.


text:
May two different component manufacturers have the same component maintenance manual number?
Is it allowed for two different component manufacturer to have the same Components Maintenance Manual number?

Why does this question have two IFE related tags? Is it specific to IFE, or related to it?

As nothing in the question relates to IFEs, I've removed the tags. If your question specifically requires information on IFEs, please feel free to edit the question and then update the tags.

Are you asking about ATA numbering?


text:
What would cause an aircraft (j3 piper cub) to slide backwards when going into a stall?
Fifty years ago, I took lessons at an uncontrolled airport and this happened to me, fortunately my instructor took over and as cool as a cucumber corrected the situation. His detailing of what he was doing fell on deaf ears.

define slide backwards..

This is called a "tail slide". It is an aerobatic maneuver when performed on purpose, but can be dangerous if done by accident because it causes damaging forces to control surfaces. Some aircraft if the CG is in the wrong location, will tend to tail slide before they fully stall (my C-177 is prone to this).

What was your attitude during this maneuver? Typically the entry for a tail slide is a 1/4 loop so that the longitudinal axis of the aircraft is perpendicular to the ground.  Were you rapidly changing your pitch attitude at the time, if you remember?

We were at about 1500ft , had just leveled off & going straight (no idea of compass or wind direction but only a few miles from a controlled airport w/private jets). I was told to climb 200ft.

Related

Based upon that limited information, I doubt that you entered a tail slide.  I can't say exactly what happened, but I can say that student pilot perceptions of what did happen in flights can be distorted.

This is called a "tail slide" or "whip stall". It is an aerobatic maneuver when performed on purpose, but can be dangerous if done by accident because it causes damaging forces to control surfaces. Some aircraft if the CG is in the wrong location, will tend to tail slide before they fully stall (my C-177 is prone to this). – Ron Beyer
Aerobatically, to enter a tail slide you would enter from level flight at full speed (maneuvering speed) and pull straight up (1/4 loop) and wait for your airspeed to bleed off to zero. The aircraft will then slide backwards and eventually the nose will drop. You stay in the dive until you have enough airspeed to recover.
Doing this on accident is a little more complicated, and is surprising that the J-3 could even do it. What happens is that you pull up while adding power just as you would be practicing a power-on stall, only the tail stalls before the wings do. The tail drops and the aircraft slides backwards. This is especially dangerous at low altitude because the only way to recover from a full tail-slide is to allow the nose to drop and gain airspeed in the dive.
The recovery action for an incipient tail-slide is similar to recovering from a stall. Nose down to the horizon and full power. Newer training requirements don't let the student go into a full stall, it teaches to recover the aircraft as soon as the aircraft begins to buffet, or at stall speed + 10kts. I'm not sure I agree with the new requirements but that is a topic for another question.

This post is community wiki because I don't think it's fair for me to get credit for simply copying and pasting Ron's response.

Thanks, I didn't post as an answer because I was missing the meat of the question, the OP asked about what the CFI was doing to recover. I added it to the post.

Do you have any good references on the flight mechanics/aerodynamics? In my view: When pulling up you would have a negative angle of attack on the tail (I'm assuming conventional tail), creating a pitch-up moment around the CG. If the critical negative angle of attack were to be exceeded, and the tailplane stalled, the pitch-up moment around the CG from the tail would decrease, and the nose would instead drop.

Tail Slides are aerobatic maneuvers with two recovery techniques;  stick forward, and stick back.  After the aircraft has been flown vertically until airspeed reaches zero and power is reduced, the aircraft slides backwards until the elevator ( now acting as a canard) pitches the aircraft quickly, either backward ( forward stick), or forward (back stick).  By holding this elevator input, the pilot causes the aircraft to "whip" into the down line, setting up for another maneuver, or simply recovering to level flight.  Only aircraft certificated for aerobatic flight should be used to demonstrate the Tail Slide or "Whip Stall", as the loads imposed on the elevator and tail assembly of standard category aircraft could exceed design limitations, resulting in partial or catastrophic structural failure.


text:
How does bank angle differ from roll angle with respect to stability in fixed-wing aircraft?
I would like a clarification on bank angle and how its different from roll angle with respect to to fixed wing aircraft. It is my understanding that the bank angle is a result of rotating the aircraft body to the stability frame, implying that if the angle of attack $\alpha$ and the side slip angle $\beta$ are zero then, and only then the bank and angle and roll angle are the same. Based on Stevens and Lewis Aircraft Control and Simulation (which doesn't define bank angle) the rotation from body to stability(wind) frame is given by
$$
C_{w-b} = \begin{bmatrix} &\cos\alpha \cos\beta & \sin\beta & \sin\alpha cos\beta \\ &-\cos\alpha\sin\beta & \cos\beta & -\sin\alpha\sin\beta\\ &-\sin\alpha & 0 & \cos\alpha  \end{bmatrix}
$$
Therefore, the bank angle $\mu$ I think is given by:
$$\mu \triangleq \begin{bmatrix} &\cos\alpha \cos\beta & \sin\beta & \sin\alpha \cos\beta \end{bmatrix} \begin{bmatrix} \phi \\ \theta \\ \psi\end{bmatrix} $$
where $\phi,\, \theta,\, \psi$ are standard aerospace Euler angles defined based on a 3-2-1 rotation.
My specific questions are:
1) is my understanding and calculation of bank angle correct? If not can someone point me to a good resource for this. I was surprised to be able to find a clear definition and formula quickly.
2) by multiplying the second and third row of $C_{w-b}$ with Euler angles we obtain a set of other two angles relative to stability frame. Do these angles have any names or specific role in aerospace dynamics/control?

In my experience, "bank angle" and "roll angle" mean exactly the same thing. I've only ever heard of one meaning of either term.
For completeness, here are some "plain English" definitions of all three Euler angles:
Pitch is the angle that the nose (the longitudinal axis) forms with the horizon. If the nose is pointed at the horizon, the pitch is 0°; if it's pointed straight up, the pitch is 90°; and if it's pointed straight down, the pitch is -90°.
Heading (sometimes called "yaw angle") is the horizontal direction that the nose is pointing, measured clockwise from true north.
Bank angle (sometimes called "roll angle") is the amount that the aircraft would have to roll (rotate around its longitudinal axis) in order to bring the wings level (which is to say, bring the lateral axis parallel with the horizon), with the top side of the aircraft facing above rather than below the horizon.
Pitch ranges from -90 to 90. The other two angles have a range consisting of a full circle; I think bank is usually expressed as a number from -180 to 180, and heading as a number from 0 to 360.

How would you express inverted flight in terms of pitch? Shouldn't that be a pitch angle near 180°?

@PeterKämpf Nope, inverted flight is a bank angle greater than 90°. If you're flying completely upside down with the nose pointed at the horizon, then the pitch is 0° and the bank angle is 180°.

As an industry standard and in most circumstances, when we say roll or bank, we are referring to body-axis roll. That is, the roll rate, $p$, is the y-axis component of the angular velocity, $\vec{\omega}=\begin{bmatrix}p & q & r\end{bmatrix}^T$, and the roll angle, $\phi$, is the 2nd rotation angle in the 3-2-1 Euler rotation sequence from the ground axis to the body axis. Also, in this parlance, roll angle and bank angle are completely synonymous.
What the OP is referring to is called stability-axis roll. That is, the roll motion is expressed in the stability-axis (or if a sideslip is present, in the wind-axis). We can easily transform the wind-axis angular velocity from the body-axis via:
$$\begin{bmatrix}p_w \\ q_w \\ r_w\end{bmatrix} = \begin{bmatrix} &\cos\alpha \cos\beta & \sin\beta & \sin\alpha \cos\beta \\ &-\cos\alpha\sin\beta & \cos\beta & -\sin\alpha\sin\beta\\ &-\sin\alpha & 0 & \cos\alpha  \end{bmatrix} \begin{bmatrix}p \\ q \\ r\end{bmatrix}$$
Or just in the roll axis (quite similar to what's written in the OP, but in roll rate only):
$$p_w = \begin{bmatrix}\cos\alpha \cos\beta & \sin\beta & \sin\alpha \cos\beta\end{bmatrix} \begin{bmatrix}p \\ q \\ r\end{bmatrix}$$
A pure wind-axis roll is enticing because it will not induce any change in sideslip, $\beta$, therefore not creating any adverse yaw or aerodynamic roll-yaw coupling. Of course, pure wind-axis roll is not natural and can only be realized through pilot coordination or active feedback control.
We can also define a wind-axis Euler roll angle (or bank angle), maybe call it $\mu$. But that expression is significantly more complex than the angular velocity expression above. So, no, your expression for $\mu$ is incorrect. If that's your desire, then the expression you should consider is:
$$\textbf{C}_{WE}(\mu, \theta_w, \psi_w) = \textbf{C}_{WB}(\alpha,\beta)\textbf{C}_{BE}(\phi,\theta,\psi)$$

I've just been trying to answer a very similar question, so I'll post what I've found.
It's a matter of definition what you describe as bank angle and what as roll angle and they are frequently used interchangeably. However, I have come across instances where they are defined as follows:
Roll Angle: Rotation about the aircraft body x-axis (this is the angle phi in the Euler set psi,theta,phi)
Bank Angle: Rotation about the velocity vector (this is the angle mu in the wind axes set mu, alpha, beta)
If you're working in body axes, you would use phi, theta, psi and if you are working in wind axes (as mentioned in the question), you would use mu, alpha, beta. As an example, the paper AGARD-CP-235 29 defines the two angles as follows: "The term bank angle (mu) is used to indicate rotation about the velocity vector while the term roll angle (phi) is used to indicate rotation about the aircraft x-axis"
mu and phi in these definitions are, in general, not the same.
I don't think you can use the transformation matrix as described in the question to transform angles. This works for vectors, so you need to transform a vector, then work out the angle you want from that.
If phi, theta and psi are the Euler angles taking the aircraft from aligned with the velocity vector to an arbitrary attitude, I derived the following to calculate mu.
mu=atan((sinphi.costheta)/(sinphi.sintheta.sinpsi+cosphi.cospsi))
I think this may be what the questioner is asking for.
I also have the equations for alpha and beta.
I can supply the equations for alpha and beta and the derivation if anyone is interested.
So, to directly answer the questions

Not sure if your understanding is correct but I've given my understanding here. Note I have always distinguished between stability and wind axes and I'm talking about what I call wind axes here. I've supplied the equation I think you need above.

Not answered the second question but I believe it is not valid to use the transformation matrix on angles in the way suggested.

Roll is often used as a verb describing rotation around the longitudinal axis of the aircraft (at least with pilots, especially flight instructors). If the aircraft has zero pitch elevation angle, then bank angle could be determined by the amount of roll that occurred from wings level. When at non-zero pitch elevation angle, the rolling movement is used to establish a bank angle, but roll change and bank change may not be the same. When pitched up at +90 degrees, or down at -90 degrees, then bank is no longer meaningful, but the aircraft can still roll around the longitudinal axis.
When at a pitch elevation angle that is not 90 or -90 degrees, bank is the angle between the aircraft lateral axis and what could be called the "wings level vector", typically  positive is considered right wing down. The wings level vector is the cross product of the local longitudinal axis and world Z axis (a Z axis where positive is upwards).
Consider the following image:
Near the center is a semi-transparent aircraft. The square pink column represents the longitudinal X axis, the dark green column is the Y axis out the left wing, and the cyan column is the aircraft's vertical or normal axis. (Often in aerospace engineering, the green axis is out the right wing and the vertical axis points downwards. Not in this case.) The thinner and lighter green line out the right wing represents the reverse of the green Y axis, representing the lateral axis. These axes are in a local coordinate system for the aircraft. The orange line is the product of the pink longitudinal axis and black world vertical axis. Bank angle is the angle between the lighter green lateral axis and the orange wings level axis.
At pitch elevation angles of +90 or -90 degrees, the cross product of lateral and wings level vectors is zero, and bank is meaningless. (I had a URL reference for this at one time that was a snippet of source code, but I can't find it at this time).

You may find more in the Wikipedia Flight Dynamcs page, though I have some heartburn with it.
The names for the earth to wind transform are bank angle, flight path angle and heading angle.
I don't have the Cw-b matrix handy now but have the Cb-w from an old reference:
Dynamics of Atmospheric flight by Bernard Etkin 1972, pg 117

Cbw
cos a cos b     -cos a sin b     -sin a
sin b            cos b            0
sin a cos b     -sin a sin b      cos a

I don't have Matlab installed or I would invert it for you.
If you are simulating an aircraft you should be aware that the Euler angle rotations have a problem and for certain sets of rotations a singularity will arise causing an instantaneous pi/2 change in angles. This typically only is a problem for tumbling spacecraft or wild aerobatics. If interested, you should study Euler symmetric or Euler–Rodrigues parameters and Hamilton quaternions.
Most of my references are too old but may be of interest.
For basic flight dynamics:
Dynamics of Atmospheric flight by Bernard Etkin 1972
Airplane Performance Stability and Control, Perkins and Hage 1949
These are of less interest for the full nonlinear dynamic problem:

Aerodyamics for Naval Aviators 1965
Theory of Flight R. Von Mises 1959
Automatic Control of Aircraft and Missiles, J Blakelock 1965
Aerodynamics for Engineering Students by Houghton & Brock 1972
Advanced Pilot's Flight Manual by W. Kershner 1977
Aircraft Dynamics and Automatic Control, McRuer Ashkenas & Graham, 1973

Happy Trails

please use quoteblocks for quotes, not codeblocks

This is one of semantics.  Bank is a state, whereas roll is an action; e.g., “The pilot rolled into a steep turn and held a 50° bank angle.”

"To roll" describes a motion. Roll Angle describes a state. Roll speed describes a motion.

Plane coordinate systems are typically expressed in three Axes, Longitudinal, lateral, and vertical. Longitudinal is nose to tail, lateral is wingtip to wingtip, Vertical is straight up.
Primary controls effect rotation about the axes. Primary controls are rudder, elevator and aileron.
Gyro instruments such as attitude indicator measure pitch and bank.
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf
That is rotation about the lateral and longitudinal axis.
The turn and slip indicator shows the rate of turn in degrees per second. The slip indicator shows the roll rate. Thus, it appears to this author that the turn and slip indicator provides an output that is the derivative of Attitude indicator.
Roll is the derivative of bank. Both are about the Longitudinal access. At least that’s how I read the FAA official description of these instruments. This is because the turn coordinator only shows the rate of turn and rate of roll in degrees per second. There is also a non-Gyro instrument Called the inclinometer. It is a curved tube with a ball inside. It is a tristate system in the sense that it is either coordinated, showing slip, or showing skid. This is a measure of rotation about the vertical axis, also known as yaw. Slip occurs when the nose is pointing in the opposite direction of the bank.
Cross controls result in a side slip that may enable you to straighten your aircraft during a crosswind landing. Dip the wing into the wind and use the rudder to align the nose with the centerline. This puts less stress on the landing gear than attempting to land while crabbing into the wind. The forward slip is a maneuver typically used for short field landing’s.
It enables you to lose altitude without increasing speed. This is accomplished by introducing drag. This is because we are showing the side of the airplane to the wind and the vigorous slip serves as a dive break. Over correct the rudder in the other direction and you have a skid. That is, nose to the wind and wing dipped to the wind. Now the lowered wing runs the risk of stalling first. This can lead to a dangerous spin.


text:
Why did my instructor recommend that I fly a Piper Seminole down to the runway rather than trying to flare it like a Cessna?
I have flown as a student the Piper Seminole 3 or 4 times, it was so much fun grabbing a bunch of throttle levers and pushing them all like a real throttle quadrant in a large jet aircraft, but anyway my question is this:
When landing and just nearing the end of final, when I was just about to the chevrons my flight instructors always asked me to fly the plane right down to the runway, which is different from flying 172's where I could flare right at the chevrons and try my best to get a 'greaser' and chirp the tires enabling a nice transition where I momentarily heard the faintest stall horn just before landing, enabling a quick turn off to a taxiway...why did I have to fly the twin right down before cutting throttles?

I still remember my CFI in the Comanche I was flying in... *It's not a Cessna, its not a Cessna, its not a Cessna" as we approached the runway. Apparently what he meant was "fly it into the ground, don't stall it". Flying it to the runway isn't really about the twin (I wasn't flying a twin Comanche), all the low-wing airplanes I've flown are landed like that.

Err, did you ask any of them?

Your instructor is trying to keep you alive - fly it to the ground. VMCA (v2) for the Piper Seminole is 56kias, its wise not to fly below that - including the flare. If you loose an engine and have to do a go-around in a crosswind and you have dropped your airspeed near 56kias, your chances are very poor.

Because this isn't Kerbal Space Program.

I'm guessing that is a landing technique that your CFI has developed for handling a Piper Seminole on landing.  Don't worry; everything is fine.
Most likely this is because the PA-44, like it's single engine cousin, the PA-32 and late model Cherokee Arrows rapidly loses energy and sinks quickly once the power is pulled out.  These tend to be the landing characteristics of larger, heavier aircraft.  Cutting the power too early in the approach to roundout could cause this aircraft to slow and drop rapidly leaving a neophyte student with little energy left to flare to a smooth touchdown.  The CFI is trying to make you cognizant of that fact; I'm sure he's endured plenty of firm landings by students with high time in C152s or C172s flying final at 65 KIAS, then chop power over the threshold markings (piano keys) to arrive at a good flare and touchdown; that technique won't work in larger twins and the best way to fly a shallow glide slope is to do so with power, then pull it out just as you begin the flare about 7-10 feet off the runway.  The airplane should then settle into a nice flare 1-2 feet off the runway, quickly exhaust all of its energy and touch down on the mains just as you hear the stall warning tone.
Another factor is the aerodynamic slipstream from the propellers. As wing mounted tractor engines push air over the wings behind the propeller disks, this, in turn is generating additional lift.  During a stabilized approach, this airflow become part of the lift the wing must generate in order to remain on speed at the selected rate of descent.  When the pilot pulls throttles to idle as part of the roundout process, this additional lift is terminated.  Combined with the high drag from the idling low pitch propellers, this causes the airplane to slow down and sink very rapidly; you will feel this in the seat of your pants if you pull the throttles out too early.  If a student pilot remains ignorant of this effect and does not compensate for it, you may be in for a very hard landing or worse.
The Piper aircraft, as you have found out, are designed to put up with a lot of sloppy handling from a new pilot and still shine.  The controls have all the finesse of a Mack truck and they really have to be manhandled to fly them.  IMO it's an attempt by the manufacturer to 'idiot proof' the airplane and make it safe and marketable to a wide range of pilot skill.
All tricycle gear aircraft should be flown to the roundout such that once the flare is established, the airplane should run out of energy and quit flying rapidly.  Strict adherence to a controlled approach speed of 1.3 Vso on final should be used (on small aircraft), but the rate at which the airplane slows and descends when the power is reduced to idle varies between airframes.  Some airplanes do this faster than others; each one has it's own unique handling characteristics and you will have to become familiar with them as you progress through different aircraft.
One note on you flying: for a VFR traffic pattern, aim for the numbers or use an optical glideslope (eg VASI, PAPI, etc), if available, on final until you have become proficient with flying the PA-34.  The 'chevrons' are a blastpad and frangible arrestment for stopping an overrun and are not designed to support an airplane during normal flight operations.  Touchdown too early and end up on the blastpad, it is possible the aircraft could break through and cause damage to the airframe, props or landing gear.  You're also likely to clip other objects like runway threshold lights with that kind of a landing.

I have an uncomfortable feeling that someone taught you at some point that "cutting throttles" and flare right at the chevrons meant ripping the throttle to idle, yanking the yoke back until it touches your belly button, and then holding those inputs until the wheels touch.
(Okay, maybe that was a slight exaggeration, but not much)
The point is that you can cut the throttle to idle in a Cessna over the threshold and still land on the thousand-foot markers if you're on glideslope. You can't do that in a Seminole, and if you try, you're going to quickly lose either speed or altitude or your instructor's respect for you as a pilot (or all three!)
Instead, try to make your throttle reduction happen at about the same rate as your flare. Ideally, your throttle should be hitting the idle stop at the same time that your yoke reaches the aft limit, and if you've really done well, both of those should happen at the same time that your main gear touches the ground.